Reflectance-facilitated ultrasound treatment and monitoring

ABSTRACT

Apparatus is provided for ablating cardiac tissue of a subject. The apparatus comprises an inflatable element, configured to be transthoracically placed within a pericardial region of the subject and a reflection-facilitation element, configured to deliver a fluid to the pericardial region, such that a portion of the fluid is disposed within the inflatable element, and another portion of the fluid is disposed outside of the inflatable element and inside the pericardial region. The apparatus additionally comprises at least one ultrasound transducer, configured to be transcatheterally placed in a heart chamber of the subject, and to apply ultrasound energy such that at least a portion of the applied energy is reflected by the fluid. Other embodiments are also described.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/015,951, entitled “REFLECTANCE-FACILITATED ULTRASOUNDTREATMENT AND MONITORING,” filed Jan. 28, 2011, which published as2011-0282203 to Tsoref et al., on Nov. 17, 2011, and which is acontinuation-in-part of U.S. Ser. No. 12/780,240 to Tsoref et al.,entitled “Reflectance-facilitated ultrasound treatment,” filed on May14, 2010 and issued as U.S. Pat. No. 8,617,150 to Tsoref et al., all ofwhich are incorporated by reference in their entirety.

FIELD OF THE APPLICATION

Embodiments of the present invention relate generally to treatment oftissue by application of energy thereto, and particularly to ablation ofcardiac tissue by application of ultrasound energy.

BACKGROUND OF THE APPLICATION

Atrial fibrillation is a common cardiac arrhythmia involving the atriaof the heart. During atrial fibrillation, the atria beat irregularly andout of coordination with the ventricles of the heart. Atrialfibrillation disrupts efficient beating of the heart and may result inblood clotting in the atrium leading to serious medical conditions suchas strokes.

Atrial fibrillation is generally caused by abnormal electrical activityin the heart. During atrial fibrillation, electrical discharges may begenerated by parts of the atria which do not normally generateelectrical discharges, such as pulmonary vein ostia in the atrium.Pulmonary vein isolation is a common medical procedure for treatment ofatrial fibrillation.

Ablation technologies currently include unipolar and bipolar techniques.The unipolar techniques employ various energy sources, includingradiofrequency (RF), microwave, high intensity focused ultrasound(HIFU), laser, and cryogenic energy sources. The bipolar techniquesemploy RF energy.

SUMMARY OF APPLICATIONS

In some embodiments of the present invention, methods and apparatus areprovided for application of ultrasound energy to tissue within a body ofa subject. For some applications, the ultrasound energy is applied totreat cardiac arrhythmias, such as atrial fibrillation, ventricularfibrillation, and/or ventricular tachycardia. During a minimallyinvasive procedure, an ultrasound tool is advanced into an organ of thebody, such as a heart chamber. The ultrasound tool comprises at leastone ultrasound transducer that is configured to transmit treatmentenergy, e.g., high intensity focused ultrasound (HIFU), towardsmyocardial tissue, and in particular towards sites within myocardialtissue which are involved in triggering, maintaining, or propagatingcardiac arrhythmias, e.g., in the case of atrial fibrillation, pulmonaryvein ostia. The treatment energy applied to the myocardial tissue causesablation of the tissue. As a result of the ablation, scars typicallyform in the ablated areas. The scars generally block abnormal electricalpulses generated in the pulmonary vein ostia from propagating into theheart chambers, thereby electrically isolating the pulmonary veins fromthe atrium and preventing cardiac arrhythmias.

For some applications, prior to application of the treatment energy, areflection-facilitation element is placed at an extramyocardial site, ina vicinity of the myocardial tissue designated for treatment. Thereflection-facilitation element provides a reflective region in theextramyocardial site. Typically, the extramyocardial site is within a“pericardial region,” which, as used in the present application,including the claims, consists of one or more regions selected from thegroup consisting of: a region between the pericardium and themyocardium, a region between the visceral pericardium (also known as theepicardium) and the parietal pericardium, and a region outside thepericardium and in contact therewith. The treatment energy applied bythe ultrasound transducer to the sites in the myocardial tissue isreflected from the extramyocardial reflective region back through themyocardial tissue. The treatment energy is thus directed at themyocardial site from two opposing directions, nearly doubling theapplied energy, thereby resulting in enhanced ablation of the myocardialtissue. This technique enables the rapid formation of an effectivetransmural lesion having an increased depth within the myocardium (asviewed from within the heart) and/or increased homogeneity along thedepth, compared to that which would be achieved in the absence of thereflection of the ultrasound energy.

For some applications, the reflection-facilitation element comprises agas-delivery element, which provides the reflective region by deliveringa gas to the extramyocardial site. The gas-delivery element, e.g., aneedle, is typically inserted through the pericardium and is configuredto deliver gas to create a gas-filled pocket within the pericardialregion, as defined hereinabove. The gas has a lower density than that ofthe surrounding tissue within the body, thereby creating a change inacoustic impedance. Due to the change in acoustic impedance, ultrasoundwaves which reach the gas are reflected. Thus, the gas in thegas-inflated extramyocardial site serves as a reflector for theultrasound energy. Typically, following inflation of the pericardiumwith gas, ultrasound energy is applied by the ultrasound transducer inthe heart to the designated treatment site in the myocardial tissue thatis adjacent to the gas-filled pericardium. The emitted energy reachesthe designated treatment site and is reflected by the gas, such that thereflected ultrasound energy passes again through the treatment site.

There is therefore provided, in accordance with some applications of thepresent invention, a method including:

providing a reflective region at a far side of tissue of a subject;

assessing whether the reflective region is in a desired location, bymeans of acoustic sensing; and

in response to assessing that the reflective region is in the desiredlocation, activating an ultrasound transducer to ablate the tissue byapplying ultrasound energy to a near side of the tissue, such that atleast a portion of the transmitted energy is reflected by the reflectiveregion onto the tissue of the subject.

For some applications, assessing includes:

applying non-ablating ultrasound energy to the near side of the tissue,such that at least a portion of the applied energy is reflected onto thetissue by the reflective region; and

monitoring an ultrasound parameter of the reflected energy.

For some applications, monitoring the ultrasound parameter includesmonitoring an amplitude of the ultrasound energy reflected by thereflective region.

For some applications, the ultrasound parameter is selected from thegroup consisting of: a scatter intensity of the reflected ultrasoundenergy, sub-harmonics of the reflected ultrasound energy, second andhigher harmonic reflections of the reflected ultrasound energy, anattenuation of the reflected ultrasound energy, and a non-linearparameter of the reflected ultrasound energy, and monitoring theultrasound parameter includes monitoring the selected ultrasoundparameter.

For some applications, assessing includes receiving sound generated bythe providing of the reflective region.

For some applications, assessing includes determining whether thereflective region is within a pericardium of the subject.

For some applications, providing the reflective region includestransthoracically advancing a reflection-facilitation element toward thedesired location.

For some applications, providing the reflective region includestransvenously advancing a reflection-facilitation element toward thedesired location.

For some applications, the desired location is within a pericardialregion of the subject that consists of one or more regions selected fromthe group consisting of: a region between the pericardium and themyocardium, a region between a visceral pericardium and a parietalpericardium, and a region outside the pericardium and in contacttherewith, and providing the reflective region includes providing thereflective region within the pericardial region.

There is further provided, in accordance with some applications of thepresent invention, a method including:

advancing into a heart chamber of a subject, an ultrasound tool thatincludes at least one ultrasound transducer;

advancing a reflection-facilitation element towards an extramyocardialsite of a subject;

operating the reflection-facilitation element to release areflection-facilitation agent to provide a reflective region at theextramyocardial site of the subject;

activating the ultrasound transducer to apply ultrasound energy tomyocardial tissue of the subject such that at least a portion of thetransmitted energy is reflected by the reflective region onto theultrasound transducer; and

monitoring an ultrasound parameter of the reflected energy.

There is still further provided, in accordance with some applications ofthe present invention, a method including:

during a first time period, activating an ultrasound transducer to applyhigh intensity ultrasound energy to the tissue site, capable of ablatingthe tissue;

during a second time period, subsequent to the first time period,activating the ultrasound transducer to apply low intensity ultrasoundenergy to the tissue site such that at least a portion of thetransmitted energy is reflected by the tissue onto the ultrasoundtransducer; and

monitoring an ultrasound parameter of the reflected energy.

For some applications the method includes, the step of performing ananalysis of the ultrasound parameter, and, responsively to the analysis,determining a level of ablation of the tissue site.

For some applications the method includes, the step of performing ananalysis of the ultrasound parameter, and, responsively to the analysis,determining a continuity of an ablation lesion throughout the tissuesite.

For some applications, monitoring the ultrasound parameter includesmonitoring amplitude of the ultrasound energy reflected by thereflective region.

For some applications, the ultrasound parameter is selected from thegroup consisting of: a scatter intensity of the reflected ultrasoundenergy, sub-harmonics of the reflected ultrasound energy, second andhigher harmonic reflections of the reflected ultrasound energy, anattenuation of the reflected ultrasound energy, and a non-linearparameter of the reflected ultrasound energy, and monitoring theultrasound parameter includes monitoring the selected ultrasoundparameter.

There is additionally provided, in accordance with some applications ofthe present invention, apparatus including an ultrasound monitoringsystem, which includes:

a reflection-facilitation element, configured to be advanced towards anextramyocardial site of a subject, and to release a reflectionfacilitation agent to provide an extramyocardial reflective region; and

an ultrasound tool, which includes at least one ultrasound transducerconfigured to be positioned within a heart chamber of the subject, andto apply ultrasound energy to myocardial tissue such that at least aportion of the transmitted energy is reflected by the reflective regiononto the myocardial tissue.

There is yet additionally provided, in accordance with some applicationsof the present invention, apparatus for monitoring ablation of a tissuesite, the apparatus including:

an ultrasound tool, which includes at least one ultrasound transducerconfigured to be positioned within a heart chamber of a subject, andconfigured to apply ablating ultrasound energy to the tissue site duringa first time period, and to apply non-ablating ultrasound energy to thetissue site, such that at least a portion of the transmitted energy isreflected by the tissue onto the ultrasound transducer during a secondperiod of time; and

a processor configured to monitor an ultrasound parameter of thereflected energy.

For some applications, the processor is further configured to perform ananalysis of the ultrasound parameter, and, responsively to the analysis,to determine a level of ablation of the tissue site.

For some applications, the processor is further configured to perform ananalysis of the ultrasound parameter, and, responsively to the analysis,to determine a continuity of an ablation lesion throughout the tissuesite.

For some applications, the processor is further configured to perform ananalysis of a plurality of ultrasound parameters, and, responsively tothe analysis, to determine a continuity of an ablation lesion throughoutthe tissue site.

For some applications, the ultrasound parameter includes an amplitude ofthe ultrasound energy reflected by the reflective region, and theprocessor is configured to monitor the amplitude of the ultrasoundenergy reflected by the reflective region.

For some applications, the ultrasound parameter is selected from thegroup consisting of: a scatter intensity of the reflected ultrasoundenergy, sub-harmonics of the reflected ultrasound energy, second andhigher harmonic reflections of the reflected ultrasound energy, anattenuation of the reflected ultrasound energy, and a non-linearparameter of the reflected ultrasound energy, and the processor isconfigured to monitor the selected ultrasound parameter.

The present invention will be more fully understood from the followingdetailed description of embodiments thereof, taken together with thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B are schematic illustrations of an ultrasound ablation system,in accordance with some applications of the present invention;

FIGS. 2A-F are schematic illustrations of the use of the ultrasoundsystem of FIGS. 1A-B for application of ultrasound energy to tissue, inaccordance with some applications of the present invention;

FIGS. 2G-J are schematic illustrations of alternative configurations ofthe system of FIGS. 1A-B, in accordance with respective applications ofthe present invention;

FIGS. 3A-C are schematic cross-sectional views of the atria showingoperation of the system for application of ultrasound energy to tissueof the left atrium, in accordance with some applications of the presentinvention;

FIGS. 4A-B are graphs showing changing parameters in cardiac tissueresulting from heating of the tissue, as determined by simulatedultrasound monitoring, in accordance with some applications of thepresent invention;

FIG. 5 is a schematic illustration of an alternative configuration ofthe system of FIGS. 1A-B, in accordance with an application of thepresent invention; and

FIGS. 6A-D are schematic illustrations of the use of the ultrasoundsystem of FIGS. 1-3 for monitoring application of ultrasound energy totissue, in accordance with some applications of the present invention.

DETAILED DESCRIPTION OF APPLICATIONS

Reference is made to FIGS. 1A-B, which are schematic illustrations of anultrasound ablation system 10, in accordance with some applications ofthe present invention. Ablation system 10 comprises an ultrasound tool20 and a reflection-facilitation element 12, which, as describedhereinbelow, provides a reflective region. Ultrasound tool 20 comprisesat least one ultrasound transducer 40. Tool 20 typically furthercomprises a catheter 22, for facilitating advancement of the tool into achamber of a heart of a subject. Tool 20 also may comprise a proximalshaft 24, which may house a distal shaft 23, which comprises a proximalportion 31, a distal portion 32, and a hinge 44. The hinge connects theproximal and distal portions, and facilitates rotation of differentelements of tool 20. (In this context, in the specification and in theclaims, “proximal” means closer to the orifice through which the tool isoriginally placed into the body, and “distal” means further from thisorifice.) It is to be noted that hinge 44 is provided by way ofillustration and not limitation. Any suitable element that mayfacilitate rotation and/or a configurational change of tool 20 may beused, e.g., a spring or a bendable portion of tool 20.

For some applications, distal portion 32 comprises an arm 30 that iscoupled to hinge 44. Arm 30 typically comprises, at a distal endthereof, the at least one ultrasound transducer 40. Tool 20 may comprisea plurality of arms 30 and any number of ultrasound transducers 40. Forsome applications, ultrasound transducer 40 is coupled to an element oftool 20 other than the arm.

For some applications, ultrasound tool 20 further comprises an anchoringelement 48, which is configured to temporarily stabilize the tool duringapplication of the treatment energy. For example, the anchoring elementmay temporarily anchor the distal end of tool 20 in a pulmonary vein.For some applications, as shown in the figures, anchoring element 48comprises an inflatable element 50, e.g., comprising a balloon, whichmay be coupled to the distal end of distal portion 32 of shaft 23.Optionally the inflatable element is shaped so as to provide a passagetherethrough for blood flow, such as described hereinbelow withreference to FIG. 5. Alternatively or additionally, for someapplications, anchoring element 48 comprises a mechanical anchoringelement. For example, the mechanical anchoring element may comprise aflexible metal element (e.g., comprising Nitinol) configured to engagethe walls of the pulmonary vein, without blocking blood flow. Forexample, the metal element may have a U-shape or J-shape, such asprovided on the Pulmonary Vein Ablation Catheter® (PVAC®) (MedtronicAblation Frontiers LLC, Carlsbad, Calif.), or a flower-shaped element,such as provide by the Multi-Array Septal Catheter® (MASCO) (MedtronicAblation Frontiers LLC, Carlsbad, Calif.).

Tool 20 is shown in FIG. 1A in a collapsed state thereof. In itscollapsed state, tool 20 assumes a smaller dimension than in itsexpanded, operative state. Thus, in its collapsed state, the tool isconfigured for insertion into a blood vessel in a location remote fromthe heart and for advancement within a chamber of the heart.Accordingly, inflatable element 50 is shown in FIG. 1A in a deflatedstate.

Reference is now made to FIG. 1B, which shows tool 20 in the expanded,operative state thereof. Tool 20 is typically configured to be deliveredto a location designated for treatment within a body of a subject, e.g.,a chamber of the subject's heart. Once delivered to the location, tool20 is transformed into the operative state as shown in FIG. 1B. In theoperative state, hinge 44 typically facilitates deflection of arm 30,such that arm 30 is deflected laterally as indicated by arrow 12A (e.g.,by 90 degrees, as shown) from a position that is aligned with alongitudinal axis of tool 20. Arm 30 may be deflected at any angle up to180 degrees, such that ultrasound transducer 40 is aimed at any desiredtreatment site. For some applications, ultrasound transducer 40 isconfigured to emit high intensity focused ultrasound (HIFU) wavestowards a target tissue. Optionally, a control wire 15 is controllableby a physician in order to adjust the angle of arm 30.

For some applications, distal portion 32 comprises a telescopicallycollapsible and extendable element 34, which facilitates the telescopicextension and collapse of distal portion 32.

Inflatable element 50 is shown in FIG. 1B in its inflated state.

Reference is made to FIGS. 2A-G, which are schematic illustrations of asystem 10 for application of ultrasound energy to tissue within a bodyof a subject, in accordance with some applications of the presentinvention. Tool 20 is configured for treatment by ultrasound energy of aregion within a body of a subject. Typically, tool 20 is configured forablating tissue, e.g., cardiac tissue. For some applications, tool 20 isinserted into a chamber of the heart and disposed in an area that isadjacent to an orifice of a blood vessel 80, e.g., adjacent to apulmonary vein ostium in the left atrium of the heart. Tool 20 isconfigured to ablate tissue in a vicinity of the orifice of the bloodvessel in order to electrically isolate the blood vessel.

FIG. 2A shows tool 20 being advanced to a location within the heart thatis adjacent to an orifice of a blood vessel. The tool is advanced in acollapsed state thereof, as described hereinabove with reference to FIG.1A.

FIGS. 2B-C show the opening of tool 20 to an operative state. Tool 20 isshown disposed within a chamber of the heart, e.g., the left atrium.Tool 20 is located adjacent to an orifice of a blood vessel 80, e.g., apulmonary vein ostium, and to cardiac tissue, e.g., an atrial wall 100.Hinge 44 typically facilitates deflection of arm 30 such that arm 30 isdeflected laterally, as indicated by arrow 12A (e.g., by between 30 and90 degrees, such as by 45 degrees, as shown) from a position that isaligned with a longitudinal axis of tool 20. The angle of deflection ofarm 30 is typically controllable by the physician during a procedure.Deflection of arm 30 brings ultrasound transducer 40 into proximity of(e.g., in contact with or within a few millimeters of) the endocardiumof cardiac tissue designated for ablation treatment, such thatultrasound transducer 40 is aimed at the designated site. There isgenerally no need for firm contact between ultrasound transducer and theendocardium. In some applications, the site designated for treatment iscardiac tissue in the atrial wall surrounding an orifice of a bloodvessel, e.g., a pulmonary vein ostium.

For some applications, distal portion 32 of tool 20 is telescopicallyextended into blood vessel 80 in the direction indicated by an arrow13A. For some applications, tool 20 comprises inflatable element 50,e.g., a balloon, coupled to the distal end of distal portion 32. Forapplications in which tool 20 comprises anchoring element 48, duringopening of tool 20 into an operative state, and subsequent applicationof treatment energy, the anchoring element stabilizes the tool againstthe wall of blood vessel 80 during application of treatment energy androtation of arm 30 or another element of tool 20 (describedhereinbelow). For example, for applications in which anchoring element48 comprises inflatable element 50, inflatable element 50 is inflated(by filling the inflatable element with fluid, i.e., a gas or a liquid)to apply pressure to a wall of blood vessel 80, in order to stabilizeand maintain tool 20 in place. Inflatable element 50 may be inflatedprior to deflection of arm 30, so as to stabilize and maintain tool 20in place during the deflection of arm 30 and subsequent application oftreatment energy. For some applications, inflatable element 50 comprisesan annular inflatable element that surrounds a distal portion of tool20.

It is to be noted that an inflation conduit 7 is coupled at a distal endthereof to inflatable element 50, and extends through a lumen of shaft23 and toward distal portion 32 of tool 20. When the operating physiciandesires to inflate element 50, fluid (i.e., a gas or liquid) isdelivered via the conduit toward inflatable element 50 from a fluidsource that is disposed outside the body of the subject. The fluid maybe pressurized.

Reference is still made to FIG. 2C. For some applications, prior toapplication of energy, reflection-facilitation element 12 is placed atan extramyocardial site, in a vicinity of the myocardial tissuedesignated for treatment. The reflection-facilitation element provides areflective region in the extramyocardial site. For some applications,the reflection-facilitation element comprises a gas-delivery element 90which delivers a gas to the extramyocardial site, e.g., within thepericardial region, as defined hereinabove. Gas-delivery element 90,which may, for example, comprise a needle, is typically inserted into orthrough the pericardium, and is configured to deliver gas to create agas-filled pocket within the pericardial region, as defined hereinabove.For some applications, needle 90 is inserted through the central port,under the collarbone. Optionally, a small camera is inserted with theneedle to provide image guidance during the insertion procedure.

Reference is made to FIG. 2D, which shows system 10 positioned forapplying ablating treatment to a target site in atrial wall 100. Thepericardial region is shown in an inflated state, with gas having beendelivered to the region between pericardium 70 and the myocardium.Alternatively, the pericardial region may be inflated by delivering thegas to the region between the visceral and parietal pericardial layers(configuration not shown). Arm 30 of tool 20 is deflected such thattransducer 40 is aimed at a target site in atrial wall 100 designatedfor treatment. Additionally, for some applications, anchoring element 48is deployed to stabilize and maintain tool 20 in place during subsequentapplication of treatment.

FIG. 2E is a schematic illustration of system 10 being operated to treatthe subject. Ultrasound transducer 40 typically transmits high intensityfocused ultrasound waves, directly heating the tissue in the acousticfocal volume (which may be cigar-shaped). For some applications,ultrasound energy emitted by transducer 40 is focused by using a curvedpiezoelectric element and/or by using a lens and/or by using a pluralityof ultrasound transducers 40. A focal point of transducer 40 istypically located in atrial wall 100, and the treatment energytransmitted by transducer 40 is generally capable of ablating myocardialtissue in atrial wall 100. For other applications, ultrasound transducer40 transmits non-focused ultrasound waves. For some applications,ultrasound transducer 40 is configured to transmit power at at least 10watts, and/or less than 100 watts, e.g., between 10 and 100 watts, e.g.,between 15 and 50 watts. Ablating ultrasound waves are shown passingthrough the tissue to reach a gas-filled region of pericardium 70. Forsome applications, ultrasound transducer 40 is configured to generateultrasound energy at a frequency having a value that is at least 100kHz, e.g., at least 1.5 MHz, and/or no more than 10 MHz, e.g., no morethan 5 MHz. At low frequencies (around 100-500 kHz), tissue destructionis primarily caused by cavitation, while at higher frequencies tissuedestruction is primarily caused by a thermal effect. When creating thethermal effect, it is generally desirable to elevate the walltemperature to 60-80 degrees C.

FIG. 2F shows ablating treatment energy being applied by ultrasoundtransducer 40 to a specific target site in atrial wall 100 and reachinga gas-filled region of pericardium 70. The gas is of lower density thanthe surrounding tissue in the body, thereby creating a change inacoustic impedance. Due to the change in acoustic impedance, the gasfunctions as a reflective region, similar to a mirror, along atrial wall100 and ultrasound waves which reach the gas are reflected. Thus,ultrasound waves are typically reflected from the reflective region,back through myocardial tissue in atrial wall 100, resulting intemperature elevation and enhanced ablation of the myocardial tissue.Reflection of the ultrasound energy such that it passes through thetissue for a second time achieves what may be considered a bipolareffect, thereby increasing the thermal effect of the ultrasound energy,resulting in the rapid formation of an effective, transmural,long-lasting lesion in the tissue. Typically, the transmural lesion isformed rapidly at each radial site in 0.1-20 seconds, e.g., in about onesecond.

As shown in FIG. 2F, reflected return waves pass through the tissuegenerally simultaneously with the transmitted waves, increasing theamount of energy that passes through the tissue and achieving improvedablation of the tissue. Increased ablation of the tissue near the ostiumof blood vessel 80 typically results in improved isolation of the bloodvessel 80 and reduced occurrence of cardiac arrhythmia.

As shown in FIG. 2F, tool 20 (e.g., arm 30, another element of the tool,or the entire tool) can be rotated in a direction indicated by an arrow14A (and/or in the opposite direction), such that ultrasound transducer40 can be aimed at any desired location around an orifice of bloodvessel 80. Rotation of tool allows circumferential ablation surroundingthe orifice of blood vessel 80, e.g., the pulmonary vein ostium, suchthat blood vessel 80 is electrically isolated from other areas of theheart, blocking conduction of undesired pulses from blood vessel intothe heart. Thus, tool 20 or an element thereof is typically rotated afull 360 degrees. Typically, anchoring element 48 does not rotate as arm30 is rotated. For example, a hinge may be provided at distal portion 32or at extendable element 34 that allows the rotation of arm 30 withoutthe rotation of anchoring element 48.

Typically, following the creation of the first lesion in the ablationsite in atrial wall 100, tool 20 is rotated slightly, e.g., by between 1and 10 degrees (e.g., between 2.5 and 7.5 degrees), such that ultrasoundtransducer 40 is now aimed at an adjacent location of atrial wall 100,for creation of an additional lesion. This procedure is typicallyrepeated until a 360-degree circumferential lesion surrounding theorifice of blood vessel 80 is formed. For some applications, transducer40 is rotated slowly while continuously transmitting ultrasound energy,thus creating a continuous circular lesion surrounding the orifice ofblood vessel 80. For some applications, the rotation is performedmanually by the physician performing the procedure. Alternatively, therotation is performed by a motor. For some applications, system 10comprises a control unit that senses when each individual lesion hasbeen formed (e.g., by monitoring temperature, as described hereinbelowwith reference to FIGS. 4A-B, e.g., by sensing that a desiredtemperature of 60 to 80 degrees has been obtained). Optionally, uponsensing that each lesion has been formed, the control unit drives themotor to rotate the tool or an element thereof, such that transducer 40applies energy to a subsequent location.

Reference is made to FIGS. 2G-J, which are schematic illustrations ofalternative configurations of system 10, in accordance with respectiveapplications of the present invention. For some applications, as shownin FIG. 2G, reflection-facilitation element 12 comprises a shapedacoustic reflector 120 (e.g., having a spherical, parabolic, orellipsoidal shape), which may comprise, for example, a metal. Reflector120 is typically placed at an extramyocardial site, e.g., within thepericardial region, as defined hereinabove, such as outside andtypically in contact with the pericardium. The reflector causesultrasound waves transmitted from transducer 40 to reflect back throughthe myocardial tissue, resulting in enhanced ablation of the myocardialtissue. Reflector 120 is placed facing ultrasound transducer 40, and ismoved as the transducer is rotated. Larger reflectors cover largerareas, and thus need be repositioned fewer times than smallerreflectors. For some applications, system 10 verifies proper positioningof reflector 120 by measuring the amplitude of the ultrasound echoreceived by transducer 40. The amplitude of the echo is small if thereflector is not properly positioned, and increases sharply when thereflector is properly positioned over the transducer.

Alternatively, reflection-facilitation element 12 comprises anothermaterial that has an acoustic impedance different from that of water,typically substantially different. For example, the element may comprisea sponge, an expanded polystyrene foam (e.g., Styrofoam®, Dow ChemicalCompany), or another material that contains a large amount of air.Ultrasound energy that is transmitted towards tissue of atrial wall 100is reflected due to the different acoustic impedance, such that thereturn energy waves pass again through the tissue.

For some applications, reflection-facilitation element 12 comprises amechanical surgical retractor, which is configured to separate thepericardium from the heart. The space thus created naturally fills withgas, thereby creating the reflective region. Surgical retractors arewidely available from numerous manufacturers.

Reference is made to FIG. 2H. For some applications,reflection-facilitation element 12 comprises an inflatable element 122,e.g., a balloon. The inflatable element is inserted into the pericardialregion, as defined hereinabove, typically between the pericardium 70 andatrial wall 100, or pressed against the outside of the pericardium. Theinflatable element is typically inflated with a fluid having a lowerdensity than water, such as a gas (e.g., carbon dioxide) or a mixture offluid and gas. The low-density fluid functions as the reflective regiondescribed hereinabove. Ultrasound energy that is transmitted towardstissue of atrial wall 100 is reflected due to the fluid-filled (e.g.,gas-filled) balloon, such that the return energy waves pass againthrough the tissue.

For some applications, inflatable element 122 is coupled to adouble-channeled catheter. A first one of the channels is in fluidcommunication with the interior of the inflatable element, fordelivering the fluid (gas or mixture of gas and liquid) to inflate theinflatable element. A second one of the channels is positioned in fluidcommunication with the pericardial region, typically the region betweenthe pericardium and the myocardium. The second channel is used todeliver a gas to the pericardial region. For some applications, thechannels are defined by two tubes, an inner tube positioned within anouter tube. For example, the inner tube may be in fluid communicationwith the inflatable element, and the outer tube may be in fluidcommunication with the pericardial region. For example, the outer tubemay be shaped so as to define slots therethrough, through which the gasis injected into the pericardial region. For some applications, theinflatable element helps separate the membrane of the myocardium fromthat of the pericardium, functioning as a retractor.

Reference is made to FIG. 2I. For some applications, arm 30 of tool 20comprises one or more orientation elements 130, which are configured toorient ultrasound transducer 40 perpendicular to atrial wall 100, and,optionally, to position the transducer at a fixed distance from theatrial wall. The housing of ultrasound transducer 40 is configured toarticulate with arm 30. For example, this articulation may be providedby a hinge 132 that couples the housing to the arm, or by one or moresprings that couple the housing to the arm (configuration not shown).For some applications, the orientation elements may be arrangedgenerally surrounding the ultrasound transducer, e.g., shaped like oneor more petals of a flower. For some applications, the elements comprisea metal, e.g., Nitinol.

Reference is made to FIG. 2J. For some applications, ultrasoundtransducer 40 comprises an array 140 of ultrasound elements, such as alinear array. Array 140 enables the ablation of a line, in addition to acircular lesion around the pulmonary veins. Alternatively, the line maybe ablated by moving a single transducer linearly. Alternatively oradditionally, a one- or two-dimensional array is used for beam formingand/or beam stirring.

FIGS. 3A-C are schematic cross-sectional views of the atria showingoperation of ultrasound ablation system 10 for application of energy, inaccordance with some applications of the present invention. For someapplications, system 10 is used for the treatment of atrialfibrillation. For such applications, system 10 is used to generateenhanced ablation in areas of pulmonary vein ostia in a left atrium 110,in order to electrically isolate pulmonary veins 80 from the rest of theheart. Enhanced ablation and scarring is achieved by creating areflective region in the vicinity of the tissue designated for ablation,such that ablating ultrasound waves are reflected back from thereflective region and pass again through the ablation site.

As shown in FIG. 3A, reflection-facilitation element 12 is used toprovide an extramyocardial reflective region 112, typically withinpericardium 70 or between the pericardium and the myocardium. Typically,reflection-facilitation element 12 is transthoracically delivered usingpercutaneous subxiphoid access to the epicardium. For some applications,reflection-facilitation element 12 may comprise gas-delivery element 90,as described hereinabove with reference to FIGS. 2C-F, which is used toinflate pericardium 70 with gas to create reflective region 112 whichreflects the applied ultrasound waves. Alternatively, thereflection-facilitation element may use other techniques for providingreflective region 112, such as those described herein, e.g., withreference to FIG. 2G or 2H.

FIG. 3A additionally shows transcatheter advancement of tool 20 intoleft atrium 110, and placement of tool 20 in a location adjacent topulmonary vein ostia in accordance with some applications of the presentinvention. Tool 20 is shown in a collapsed state prior to application ofenergy by transducer 40. For some applications, a transseptal approachis used to advance tool 20 to left atrium 110, using catheter 22, asshown in FIGS. 3A-C. Alternatively, tool 20 may be advanced to leftatrium 110 using a transapical approach, via the apex of the leftventricle and the mitral valve (approach not shown). Furtheralternatively, tool 20 may be advanced to left atrium 110 via the aorta,the left ventricle, and the mitral valve (approach not shown). For someapplications, tool 20 is first advanced into the left atrium, andextramyocardial reflective region 112 is subsequently provided, whilefor other applications, region 112 is first provided.

FIG. 3B shows the opening of tool 20 into an operative state within aleft atrium of the heart. Tool 20 is located adjacent to a pulmonaryvein ostium, and to tissue of atrial wall 100. Hinge 44 typicallyfacilitates deflection of arm 30, such that arm 30 is deflectedlaterally from a position that is aligned with a longitudinal axis oftool 20. Deflection of arm 30 brings ultrasound transducer 40 into theproximity of cardiac tissue designated for ablation treatment, such thatultrasound transducer 40 is aimed at the designated site.

For some applications, as mentioned above, distal portion 32 of tool 20is telescopically extended into the pulmonary vein, such that anchoringelement 48 (e.g., inflatable element 50) is disposed within a lumen ofpulmonary vein 50. Anchoring element 48 is shown comprising inflatableelement 50, which is shown inflated, applying pressure to a wall of thepulmonary vein, in order to stabilize and maintain tool 20 in placeduring application of treatment energy. Alternatively, anchoring is notprovided, or other anchoring techniques are used, such as describedherein. An exploded view of ultrasound transducer 40 shows theinitiation of treatment by applying ablating ultrasound waves to thetissue of atrial wall 100.

FIG. 3C shows the rotation of arm 30 to successively aim ultrasoundtransducer 40 at a plurality of sites on atrial wall 100, typically toform a complete circular lesion 114, thereby electrically isolatingpulmonary vein 80 from left atrium 110.

It is to be noted that system 10 can be used to treat other types ofcardiac arrhythmia such as ventricular tachycardia. For suchapplications, tool 20 is advanced into a ventricle of a subject andlesions are created by ablation of tissue in the ventricle byapplication of ultrasound energy in accordance with applications of thepresent invention.

Reference is again made to FIGS. 1A-3C. For some applications ultrasoundtransducer 40 is configured to transmit ultrasound energy that iscapable of damaging tissue by a variety of mechanisms, e.g., ablationand/or cavitation and/or standing waves or a combination thereof.

For some applications, the ultrasound HIFU energy application techniquesdescribed herein are practiced in combination with other types ofablation, such as cryoablation and/or radiofrequency (RF) ablation.

It is also to be noted that application of treatment energy to siteswithin a chamber of the heart is not limited to blood vessel orificesbut may be applied to any region in the heart which is involved intriggering or maintaining cardiac arrhythmias.

Reference is still made to FIGS. 1A-3C. For some applications,transducer 40 comprises phased array ultrasound probes which typicallytransmit ablating energy in a series of rings. For such applications,transducer 40 simultaneously ablates a 360-degree circumferential lesionsurrounding the orifice of a blood vessel, substantially withoutrotation of the transducer.

Reference is again made to FIGS. 1A-3C. Phrenic nerve damage is anundesired yet potential complication of catheter-based ablationprocedures including ablation by ultrasound energy, as described inSacher et al. (2007) (referenced above). Some applications of thepresent invention reduce the potential of damage to the phrenic nerve,e.g., to the left phrenic nerve, which is located in proximity to theleft atrial appendage. Typically, for applications in whichreflection-facilitation element 12 delivers gas to inflate thepericardium, the gas typically distances the phrenic nerve from the siteof ablation and creates a gas-filled barrier between the phrenic nerveand the ablation site, thereby protecting the phrenic nerve frompotential damage by the applied ultrasound energy.

Additionally or alternatively, some applications of the presentinvention reduce potential damage to the esophagus that may be caused byablation procedures performed on the heart. Typically, for applicationsin which reflection-facilitation element 12 delivers gas to inflate thepericardium, the gas creates a gas-filled barrier between the esophagusand the ablation site, thereby protecting the esophagus from potentialdamage by the applied ultrasound energy.

For some applications, system 10 is configured to continuously orperiodically monitor the treated tissue during treatment, in order toassess whether the ablation is sufficient. For some applications, thesystem performs the monitoring by electrical mapping of the tissue, suchthat conductance of electrical signals is mapped and the need forfurther treatment is assessed. For some applications, a multi-electrodecatheter for mapping of conductance following application of treatmentis used.

For some applications, system 10 monitors the treated tissue usingultrasound, typically to detect the temperature of the treated tissue.Various ultrasound parameters are dependent on the temperature of thetissue. For example, the speed of sound is dependent on the temperatureof the tissue. In the case of a muscle (or atrial wall), the speed ofsound increases as the temperature is elevated. Thus the time of flight(TOF) decreases as the temperature is elevated, assuming that thedistance that the sound waves travel is fixed. The beating heart is morecomplicated, since due to atrial contraction the distance changes (evenif the transducer is fixed in the same position or distance from theatrial wall). However, the distance change is predictable, and thus, forsome applications, is used to extract the temperature change, asdescribed below. For some applications, the ultrasound parameter isselected from the group consisting of: an amplitude of the ultrasoundenergy applied by the ultrasound transducer and reflected by thereflective region, a scatter intensity of the reflected ultrasoundenergy, sub-harmonics of the reflected ultrasound energy, second andhigher harmonic reflections of the reflected ultrasound energy, anattenuation of the reflected ultrasound energy, and a non-linearparameter of the reflected ultrasound energy.

Additionally or alternatively, system 10 monitors ablation of the tissueusing ultrasound. As described hereinabove, for some applications, theultrasound transducer is rotated slowly while continuously transmittingablating ultrasound energy, thus creating a continuous circular lesionsurrounding the orifice of blood vessel. As provided by someapplications of the present invention, following ablation of the tissue,the ultrasound transducer is activated to apply low intensity ultrasoundenergy to the ablated tissue such that at least a portion of thetransmitted energy is reflected by the tissue onto the ultrasoundtransducer. System 10 is configured to monitor a pattern of thereflected energy (echo) received by transducer 40. If the circularlesion is continuous, the echo pattern shows a continuous pattern withgenerally steady intensity. If the circular lesion is not continuous andgaps appear in the ablated tissue, the echo pattern presents varyingintensities.

FIGS. 4A-B are graphs showing changing parameters in cardiac tissueresulting from heating of the tissue as determined by simulatedultrasound monitoring, in accordance with an application of the presentinvention (all units are arbitrary units (AU)). For some applications,ultrasound is used to monitor the treatment. For these applications,waves reflected by a reflective region in the tissue, e.g., a gasbetween the pericardium and atrial wall, are detected by ultrasoundtransducer 40, and the time of flight (TOF) is then measured. Changes inthe time of flight (TOF) can be used as an indicator for proper heatingof the tissue, in accordance with some applications of the presentinvention.

The graph in FIG. 4A shows a dashed line representing time of flight(TOF) in cardiac tissue under normal, untreated conditions. The TOFexhibits generally sinusoidal behavior due to contractions of theatrium. As described above, as a result of heating of a muscle, e.g.,cardiac muscle, the speed of sound in the muscle typically increases.The continuous line in FIG. 4A represents (simulated) TOF in muscletissue that has been heated by ultrasound energy applied thereto. Thechanges in TOF enable ultrasound monitoring of the treatment applied, inaccordance with applications of the present invention.

The graph in FIG. 4B shows the result of signal processing of the TOFparameter, in accordance with an application of the present invention.The processing includes generating an average moving window having awidth equal to a period of the beating heart (i.e., the heart rate). Thedashed line shows a case with no heating, and the solid line shows acase that includes heating. Using an average moving window, thesinusoidal behavior is eliminated and the inclination of the TOF isobtained, thus monitoring the temperature of the ablation.

When the monitored temperature shows that the target temperature hasbeen obtained, the ultrasound transmission is ceased, and the transduceris rotated to a different radial location, either manually by thephysician, or by a motor, e.g., driven by a control unit of system 10.

For some applications, system 10 alternatively or additionally measuresother ultrasound parameters, such as the amplitude of reflectedultrasound waves, scatter intensity, sub-harmonics, second and higherharmonic reflections, attenuation and/or non-linear parameters, Thesystem uses these measured parameters as indicative of change in thetreated tissue. When sufficient change is obtained, the ultrasoundtransmission is ceased and the transducer rotated, either manually bythe physician, or by a motor, e.g., driven by a control unit of system10.

It is noted that inflatable element 50, the telescopic expansion ofdistal portion 32 of tool 20, arm 30, and the 360 rotation of tool 20are described hereinabove by way of illustration and not limitation, andthe scope of the present invention includes a system that includes onlysome, or none, of these elements.

Reference is now made to FIG. 5, which is a schematic illustration of analternative configuration of system 10, in accordance with anapplication of the present invention. In this configuration, ablationtool 20 is configured to apply ultrasound energy to a series of areas onthe heart wall from a location outside of the heart, such as against ornear an outer surface of the pericardium. Reflection-facilitationelement 12 is configured to be placed inside the left atrium, to providethe reflective region within the atrium. For example, thereflection-facilitation element may comprise an inflatable element 150,such as described hereinabove with reference to 2H, mutatis mutandis, ora shaped acoustic reflector, such as described hereinabove withreference to 2G, mutatis mutandis. Optionally, the inflatable element isshaped so as to define a passage therethrough to allow the flow ofblood.

Reference is now made to FIGS. 6A-D. For some applications, system 10 isconfigured to allow a physician to provide a reflective region at a farside of tissue of a subject and to assess whether the reflective regionis in a desired location, by means of acoustic sensing, and in response,to apply ablating energy to the tissue. For some applications, system 10is configured to transmit low intensity, non-ablating ultrasound energyin order to verify proper positioning of reflection-facilitation element12. Typically, reflection-facilitation element 12 comprises gas-deliveryelement 90, which is advanced, e.g., transthoracically, towards anextramyocardial site of a subject. System 10 is configured to monitoradvancement of gas-delivery element 90 and verify proper positioningwithin the extramyocardial site, specifically within a desiredpericardial region as described herein. Ultrasound transducer 40 isactivated to continuously apply non-ablating low intensity ultrasoundenergy to myocardial tissue, while element 90 is transthoracicallyadvanced towards the pericardial region of the subject. While beingadvanced, element 90 continuously or intermittently releases smallamounts of a gas, in order to provide reflective regions. System 10assesses the proper positioning of gas-delivery element 90 within thepericardial region and the providing of the reflective region bymeasuring the amplitude of the ultrasound echo received by transducer40. The amplitude of the echo is small if gas-delivery element 90 isremote from the pericardial region (because there is no gas yet in thepericardial region), and increases sharply when gas-delivery element 90is properly positioned in the pericardial region. Typically, in responseto assessing that the gas-delivery element and consequently thereflective region are in the desired location, the ultrasound transduceris activated to ablate the tissue as described hereinabove.

FIG. 6D is a graph showing a simulated change in echo amplitude ofreceived ultrasound waves over a time period in which the gas-deliveryelement is being advanced towards and subsequently into the pericardialregion. As shown, the amplitude of the echo increases sharply over theperiod of time which typically corresponds to penetration of thepericardial region. Additionally or alternatively, an acoustic sensingelement, e.g., an ultrasound or other transducer, functions in amicrophone mode, detecting the release of gas into the pericardialregion.

Reference is made to FIGS. 1A-3C and 5-6D. For some applications,separate ultrasound transducers are utilized for delivery ofnon-ablating and ablating energy. For example, a focusing ultrasoundtransducer may be used for delivery of ablating energy. Additionally oralternatively, flat ultrasound transducers and/or diverging ultrasoundtransducers that spread ultrasound waves and typically sense a largerarea may be used for delivery of non-ablating energy.

Reference is again made FIGS. 1A-3C and 5-6D. For some applications,reflection-facilitation element 12 comprises electrodes that create gasin order to provide a reflective region.

Although techniques of the present invention have generally beendescribed herein as being applied to cardiac tissue, these techniquesmay additionally be used, mutatis mutandis, to treat other tissue of asubject, such as liver tumors or varicose veins. The techniques are usedto provide a reflective region at a far side of the tissue, by placing areflective-facilitation element at the far side, and to ablate thetissue by applying ultrasound energy to a near side of the tissue suchthat at least a portion of the applied energy is reflected onto thetissue by the reflective region. Additionally or alternatively, thesetechniques may be used to monitor a tissue of a subject by providing areflective region at a far side of the tissue, by placing areflective-facilitation element at the far side and applyingnon-ablating ultrasound energy to a near side of the tissue, such thatat least a portion of the applied energy is reflected onto the tissue bythe reflective region.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

1. A method comprising: providing a reflective region at a far side oftissue of a subject; assessing whether the reflective region is in adesired location, by means of acoustic sensing; and in response toassessing that the reflective region is in the desired location,activating an ultrasound transducer to ablate the tissue by applyingultrasound energy to a near side of the tissue, such that at least aportion of the transmitted energy is reflected by the reflective regiononto the tissue of the subject.
 2. The method according to claim 1,wherein assessing comprises: applying non-ablating ultrasound energy tothe near side of the tissue, such that at least a portion of the appliedenergy is reflected onto the tissue by the reflective region; andmonitoring an ultrasound parameter of the reflected energy.
 3. Themethod according to claim 2, wherein monitoring the ultrasound parametercomprises monitoring an amplitude of the ultrasound energy reflected bythe reflective region.
 4. The method according to claim 2, wherein theultrasound parameter is selected from the group consisting of: a scatterintensity of the reflected ultrasound energy, sub-harmonics of thereflected ultrasound energy, second and higher harmonic reflections ofthe reflected ultrasound energy, an attenuation of the reflectedultrasound energy, and a non-linear parameter of the reflectedultrasound energy, and wherein monitoring the ultrasound parametercomprises monitoring the selected ultrasound parameter.
 5. The methodaccording to claim 1, wherein assessing comprises receiving soundgenerated by the providing of the reflective region.
 6. The methodaccording to claim 1, wherein assessing comprises determining whetherthe reflective region is within a pericardium of the subject.
 7. Themethod according to claim 1, wherein providing the reflective regioncomprises transthoracically advancing a reflection-facilitation elementtoward the desired location.
 8. The method according to claim 1, whereinproviding the reflective region comprises transvenously advancing areflection-facilitation element toward the desired location.
 9. Themethod according to claim 1, wherein the desired location is within apericardial region of the subject that consists of one or more regionsselected from the group consisting of: a region between the pericardiumand the myocardium, a region between a visceral pericardium and aparietal pericardium, and a region outside the pericardium and incontact therewith, and wherein providing the reflective region comprisesproviding the reflective region within the pericardial region.
 10. Amethod comprising: advancing into a heart chamber of a subject, anultrasound tool that includes at least one ultrasound transducer;advancing a reflection-facilitation element towards an extramyocardialsite of a subject; operating the reflection-facilitation element torelease a reflection-facilitation agent to provide a reflective regionat the extramyocardial site of the subject; activating the ultrasoundtransducer to apply ultrasound energy to myocardial tissue of thesubject such that at least a portion of the transmitted energy isreflected by the reflective region onto the ultrasound transducer; andmonitoring an ultrasound parameter of the reflected energy.
 11. A methodfor monitoring ablation of a tissue site, the method comprising: duringa first time period, activating an ultrasound transducer to apply highintensity ultrasound energy to the tissue site, capable of ablating thetissue; during a second time period, subsequent to the first timeperiod, activating the ultrasound transducer to apply low intensityultrasound energy to the tissue site such that at least a portion of thetransmitted energy is reflected by the tissue onto the ultrasoundtransducer; and monitoring an ultrasound parameter of the reflectedenergy.
 12. The method according to claim 11, further comprising thestep of performing an analysis of the ultrasound parameter, and,responsively to the analysis, determining a level of ablation of thetissue site.
 13. The method according to claim 11, further comprisingthe step of performing an analysis of the ultrasound parameter, and,responsively to the analysis, determining a continuity of an ablationlesion throughout the tissue site.
 14. The method according to claim 11,wherein monitoring the ultrasound parameter comprises monitoring anamplitude of the ultrasound energy reflected by the reflective region.15. The method according to claim 11, wherein the ultrasound parameteris selected from the group consisting of: a scatter intensity of thereflected ultrasound energy, sub-harmonics of the reflected ultrasoundenergy, second and higher harmonic reflections of the reflectedultrasound energy, an attenuation of the reflected ultrasound energy,and a non-linear parameter of the reflected ultrasound energy, andwherein monitoring the ultrasound parameter comprises monitoring theselected ultrasound parameter.
 16. Apparatus comprising an ultrasoundmonitoring system, which comprises: a reflection-facilitation element,configured to be advanced towards an extramyocardial site of a subject,and to release a reflection facilitation agent to provide anextramyocardial reflective region; and an ultrasound tool, whichcomprises at least one ultrasound transducer configured to be positionedwithin a heart chamber of the subject, and to apply ultrasound energy tomyocardial tissue such that at least a portion of the transmitted energyis reflected by the reflective region onto the myocardial tissue. 17.Apparatus for monitoring ablation of a tissue site, the apparatuscomprising: an ultrasound tool, which comprises at least one ultrasoundtransducer configured to be positioned within a heart chamber of asubject, and configured to apply ablating ultrasound energy to thetissue site during a first time period, and to apply non-ablatingultrasound energy to the tissue site, such that at least a portion ofthe transmitted energy is reflected by the tissue onto the ultrasoundtransducer during a second period of time; and a processor configured tomonitor an ultrasound parameter of the reflected energy.
 18. Theapparatus according to claim 17, wherein the processor is furtherconfigured to perform an analysis of the ultrasound parameter, and,responsively to the analysis, to determine a level of ablation of thetissue site.
 19. The apparatus according to claim 17, wherein theprocessor is further configured to perform an analysis of the ultrasoundparameter, and, responsively to the analysis, to determine a continuityof an ablation lesion throughout the tissue site.
 20. The apparatusaccording to claim 17, wherein the processor is further configured toperform an analysis of a plurality of ultrasound parameters, and,responsively to the analysis, to determine a continuity of an ablationlesion throughout the tissue site.
 21. The apparatus according to claim17, wherein the ultrasound parameter includes an amplitude of theultrasound energy reflected by the reflective region, and wherein theprocessor is configured to monitor the amplitude of the ultrasoundenergy reflected by the reflective region.
 22. The apparatus accordingto claim 17, wherein the ultrasound parameter is selected from the groupconsisting of: a scatter intensity of the reflected ultrasound energy,sub-harmonics of the reflected ultrasound energy, second and higherharmonic reflections of the reflected ultrasound energy, an attenuationof the reflected ultrasound energy, and a non-linear parameter of thereflected ultrasound energy, and wherein the processor is configured tomonitor the selected ultrasound parameter.